PROJECT SUMMARY Organogenesis relies upon the carefully coordinated regulation of collective cell movement, in which a group of cells operate as a cohesive entity, coordinating their individual trajectories to reach a common destination. Cardiogenesis, for example, employs collective cell movement during multiple phases of morphogenesis, including the assembly of the heart tube, the protrusion of trabeculae, and the construction of septae. Despite the importance of these morphogenetic processes, we do not yet understand the molecular mechanisms that govern collective cell behavior in the developing heart. In particular, the cues that control the timing and routes of cardiac cell movement remain largely mysterious. Here, we aim to decipher the genetic pathways that control collective cell movement during heart tube assembly in the zebrafish embryo. To build the heart tube, bilateral groups of cardiomyocytes move toward the midline and merge through a process called cardiac fusion. Our prior studies have suggested a model in which interactions between the myocardium, endoderm, and extracellular matrix (ECM) act to facilitate cardiac fusion. However, the elucidation of these tissue-level interactions has not answered key open questions regarding the molecular mechanisms that drive cell behavior. Notably, we do not yet know which signals dictate the direction of cardiomyocyte trajectories or which cues control the rate of cardiomyocyte mobility. It is therefore exciting that we will investigate two novel regulators of cardiomyocyte movement ? the platelet-derived growth factor receptor Pdgfra and the transmembrane protein Tmem2 ? that are poised to address these unresolved issues. First, to test the hypothesis that medially-located PDGF ligands activate Pdgfra in cardiomyocytes and thereby control the direction of cardiomyocyte movement, we will (a) employ time-lapse analysis to pinpoint the impact of pdgfra on myocardial cell behavior, (b) use tissue-specific transgenes to determine where pdgfra acts to influence cardiac fusion, (c) test whether PDGF ligands act as directional cues for myocardial movement, (d) identify effector pathways acting downstream of Pdgfra in this context, and (e) evaluate whether Pdgfra plays a comparable role during cardiac fusion in mouse. Second, to test the hypothesis that the Tmem2 ectodomain facilitates an efficient rate of myocardial motility through modulation of the ECM, we will (a) employ time-lapse analysis to determine the influence of tmem2 on myocardial cell behavior, (b) determine whether tmem2 has a non-autonomous effect on myocardial movement, (c) test whether Tmem2 regulates cardiac fusion by modulating the ECM, and (d) utilize structure-function and proteomic analyses to identify which domains of Tmem2 are required for its function and which proteins interact with these domains. Together, these studies will reveal essential mechanisms of heart tube assembly, uncover new paradigms for the regulation of collective cardiac cell movement, shed light on the origins of congenital heart disease, and facilitate future tissue engineering approaches for cardiac repair.